Aromatic and heteroaromatic aldehydes and carboxylic acids, such as 4-nitro-2-sulfobenzaldehyde or -benzoic acid, are intermediates in the preparation of herbicides and plant growth regulators.
DE 42 36 902 describes the preparation of 4-nitro-2-sulfobenzoic acid via KMnO4 oxidation of 4-nitrotoluene-2-sulfonic acid. However, this route is not suitable for an environmentally friendly large-scale industrial preparation of the desired compound. This is also true for other known oxidation methods, for example those involving sodium hypochlorite and nickel peroxide. Helv. Chimica Acta 15, 576, 583 (1932) discloses the use of 4,4xe2x80x2-dinitrostilbene-2,2xe2x80x2-disulfonic acid as starting material. However, once more the oxidation is carried out using KMnO4.
For this reason, novel synthesis routes for preparing aromatic and heteroaromatic carboxylic acids and aldehydes are investigated by which routes the desired end products can be prepared in an environmentally friendly manner even on a large industrial scale.
Accordingly, the present invention provides a process for preparing substituted aromatic and heteroaromatic aldehydes or carboxylic acids of the formula 
in which R1 and R2 are H, C1-C4 alkyl, C1-C4 alkoxy, OH, NO2, CN, COOH, CONH2, COOR, NH2, SO3H or halogen and R is C1-C4 alkyl and where R1 and R2 are not simultaneously H, X is C or N and Z is CHO or COOH, which comprises dissolving a corresponding symmetric stilbene compound of the formula 
in which R1, R2 and X are as defined above either directly or as a salt in water and reacting it at a temperature of from 0 to +100xc2x0 C. with ozone, giving a mixture comprising the corresponding aldehyde of the formula I and hydroperoxide from which
a) after acidic or alkaline decomposition of the hydroperoxide, the corresponding aldehyde of the formula I is isolated, or which
b) is directly reacted, under acidic or alkaline decomposition of the hydroperoxide and with addition of an oxidizing agent, to give the corresponding carboxylic acid of the formula I or its salt, which is then isolated.
The process according to the invention is suitable for preparing compounds of the formula I.
These compounds can be aromatic or heteroaromatic aldehydes or carboxylic acids. Accordingly, the substituent Z is CHO or COOH. In the o, m or p position to this substituent, a C atom of the aromatic ring can be replaced by an N atom, so that X can be C or N. The ring furthermore carries the substituents R1 and R2, which can likewise be positioned in the o, m or p position to Z. If one of these positions is replaced by X=N, the N atom does not have any substituents.
In the formula I, R1 and R2 are H, C1-C4 alkyl, C1-C4 alkoxy, OH, NO2, CN, COOH, CONH2, NH2, COOR, SO3H or halogen, such as Cl, Br, F.
R can be C1-C4 alkyl. However, R1 and R2 may not simultaneously be H. R1 is preferably NO2 and R2 is preferably SO3H.
Thus, examples of the compounds which can be prepared by the process according to the invention are 2-sulfobenzoic acid, 4-nitro-2-sulfobenzoic acid, 4-nitro-2-sulfobenzaldehyde, 4-aminobenzoic acid, terephthalic acid, substituted pyridine carboxylic acids, such as 2,3-pyridinedicarboxylic acid, etc., or their Na or K salts.
Preference is given to preparing 4-nitro-2-sulfobenzaldehyde or -benzoic acid, or the Na or K salt of 4-nitro-2-sulfobenzoic acid.
The starting materials used are the corresponding symmetric stilbene compounds of the formula II in which the substituents R1 and R2 are as defined for the compounds of the formula I.
Furthermore, if heteroaromatic end products are to be prepared, in each case one C atom in the rings is replaced by an N atom.
Suitable compounds of the formula II are employed directly or in the form of their salts.
Suitable salts are, for example, Na or K salts of acids or chlorides, nitrates, sulfates, phosphates of basic groups.
Examples of suitable starting materials are, accordingly, stilbenes such as stilbene-2,2xe2x80x2-disulfonic acid, 4,4xe2x80x2-dinitrostilbene-2,2xe2x80x2-disulfonic acid, 4,4xe2x80x2-vinylenedianiline, 4,4xe2x80x2-vinylenedipyridine, 4,4xe2x80x2-stilbenedicarboxylic acid, etc., or salts of these compounds.
The starting materials are dissolved in such an amount of water that a 1-50% strength, preferably a 5-15% strength, solution is obtained.
If the starting material itself is not water-soluble, an aqueous solution of the corresponding salt can be prepared by adding suitable bases or acids.
The resulting solution is treated with a gaseous ozone-carrying O2 stream sufficiently long for the equivalent amount of ozone or an excess to be taken up.
The end and thus the duration of the reaction is defined by the consumption of the theoretical amount of ozone and can easily be determined by a simultaneously occurring increased ozone breakthrough.
The end of the reaction can furthermore easily be determined by suitable in-process monitoring of the consumption of the starting material.
The temperature of the ozonolysis is from 0 to 100xc2x0 C. Preference is given to a temperature of from +5 to +80xc2x0 C., particularly preferably from +15 to +70xc2x0 C.
After the ozonolysis has ended, the mixture comprises equimolar amounts of the corresponding aldehyde of the formula I and the corresponding hydroperoxide.
The hydroperoxide is then subjected to acidic or alkaline decomposition. Accordingly, the reaction mixture is, after the ozonolysis, heated in aqueous acidic solution, or an aqueous solution of a base is added until no more peroxide can be detected in the reaction mixture.
After the decomposition, the hydroperoxide can give rise either to the corresponding acid or to the aldehyde.
If the aldehyde is the desired end product, it is isolated from the mixture. The formation of aldehyde or acid from the hydroperoxide depends on the substituents of the compound of the formula I.
Preference is given to isolating an aldehyde of the formula I which, as substituent R1, has NO2 and, as substituent R2, has H or SO3H.
To isolate the aldehyde, the hydroperoxide is preferably subjected to an alkaline decomposition. To this end, particular preference is given to using aqueous sodium hydroxide solution, which is added until an alkaline pH, preferably of more than pH 8, is reached.
The mixture is then cooled and the aldehyde is crystallized out.
If the desired end product is the corresponding carboxylic acid, an oxidizing agent is immediately, without isolation of the aldehyde, added to the reaction mixture which is obtained after the ozonolysis and which comprises aldehyde and hydroperoxide compound.
Suitable oxidizing agents are, for example, hydrogen peroxide, peracids, ozone, molecular oxygen, sodium hypochlorite or sodium perborate. Preference is given to using hydrogen peroxide or a peracid. The oxidizing agent is added in an amount which is at least equimolar to the amount of aldehyde present in the reaction mixture, i.e. in an amount which is at least 0.5 molar, based on the stilbene employed. The temperature of the oxidation varies depending on the oxidizing agent used and is in a temperature range of from 0 to 100xc2x0 C.
The acidic or basic decomposition of the hydroperoxide can be carried out either before or after the oxidation, but also at the same time as the oxidation. Preference is given to subjecting the hydroperoxide to acid decomposition.
After the oxidation has ended, the corresponding carboxylic acid or its salt, preferably the Na or K salt, is crystallized, for example either directly by crystallization from the aqueous reaction mixture, for example by concentration, cooling or addition of a suitable precipitating agent, or first extracted with an organic solvent, followed by crystallization of the target product from the extract.
If the starting material used is the sodium salt of the corresponding compound and if the desired end product is the K salt of the carboxylic acid, this is precipitated, for example, by addition of potassium salts, such as potassium chloride, aqueous potassium hydroxide solution, potassium formate, potassium acetate, potassium carbonate, etc.
The mother liquor which is obtained when the reaction mixture is worked up can, after concentration, be recycled into a new oxidation batch. This is advantageous, in particular, if the peroxide decomposition is carried out using acetic acid, since, together with the mother liquor, the acetic acid is recycled, too.
The process according to the invention affords the desired aldehydes and carboxylic acids, if appropriate in the form of their salts, in an environmentally friendly way, since hardly any waste is produced owing to the use of symmetric stilbenes as starting material.
The target products can be obtained in high purity and in high yields.
The process according to the invention is particularly suitable for preparing 4-nitro-2-sulfobenzaldehyde or -benzoic acid or the sodium or potassium salt of this compound.